Yogurt Monograph Series: Concentrating Yogurt
Daniel Wilbanks, Technical Contributor
Filtration has been used to concentrate cultured milks for thousands of years. Concentrated yogurts – such as Greek yogurt, labneh, and skyr – were historically produced by straining yogurt through a cloth. Whey passes through the cloth while the white mass is retained, increasing the protein and fat content in the yogurt.
Traditional modern yogurt concentration begins with a white mass cultured in a large tank, which is stirred and concentrated using cloth or a filtration system such as
ultrafiltration (UF) or plate-and-frame. Straining through cloth is similar in principle to other filtration processes in that portions of the white mass are concentrated based on the physical size of the components (Figure 1), similar to straining pasta.
Cloth straining is an ancient but inefficient technique referred to as dead-end filtration because the product flows in the same direction as the permeate. This causes large particles to block pores, reducing the rate at which whey is removed – referred to as flux. In contrast, plate-and-frame and UF push feed across the filter (cross-flow) which dramatically improves flux. Cross-flow filtration may be used before or after culturing. Mechanical separators may also be used to concentrate yogurt, but only after culturing. Separators are fundamentally different from filtration in that they separate streams based on differences in density (Figure 2), producing a heavy (protein) and light (fat and water soluble components) phase.
Because the milk is heated at high temperatures before culturing yogurt, whey proteins are denatured and coagulate during fermentation. Hence, they are not found
in the whey of yogurt. The absence of whey proteins in yogurt whey (acid whey) is a major point of distinction between it and cheese whey (sweet whey). True to its
name, acid whey is more acidic and so has a high mineral content.
The concentration of calcium in milk exceeds its solubility limit. Calcium in excess of its solubility would typically precipitate, like a kidney stone. To avoid mineral deposits in milk, nature has cleverly “stored” most of the calcium in milk within the casein proteins. While stored, this insoluble calcium is inert which allows for the remarkably high calcium content in milk. If milk is concentrated before culturing it retains most of the calcium stored within the casein. However, insoluble calcium is dissolved in the presence of sufficient acid (pH~5.3). So, yogurt concentrated after culturing will have a lower mineral content because the calcium is dissolved and removed with the whey (Figure 3).
At high levels, calcium imparts a chalky, dry mouth feel. So, while concentration of milk by UF prior to culturing can achieve a target protein level without producing acid whey, manufacturers should consider the high calcium content of this process. Because of the low protein and high calcium content in acid whey, there is low demand for it and many manufacturers struggle with handling the large amounts of this co-product generated. For this reason, a combination of concentrating before culturing, after culturing, and/or inclusion of dairy solids (such as MPC or WPC) is sometimes used.
This article originally appeared in the Winter 2024 issue of the Dairy Pipeline.